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MG Midget and Sprite Technical - Fuel guage adjustment

Is there a way of adjusting an early type fuel gauge?
It accurately shows empty, when it is !
(If the float sender is wrongly orientated when fitted they dont indicate below 1/4 full, even when empty. BTDT)
But when full ( just brimmed it at £1-52/litre!) it registers exactly 1/2 full.
Its an early type with no voltage stabiliser. Is there a way of adjusting it?
Or alternatively, is there an electrickery method such as adding a resistor into the feed wire, or some such magic? If so, anyone let me in on the magic method?

And no, I am not going to renumber the gauge dial !
GuyW

When ours did that there was a small amount of fuel in the tank unit's float.
Paul Walbran

I don't think it's that Paul. Though it isn't impossible. The car has only fairly recently been fully refurbished and the sender and float appeared fine at the time.

I've a recollection of possibly reading about adjustment screws on the back of the gauge. I've not inspected for that yet but would have expected that to be for fine tuning rather than a reading 100% out.
GuyW

Personally I would not touch the gauge.Its normally the sender. It sounds like a tank out and see if the gauge moves through the full arc (E to F) when moving the sender arm.
Bob Beaumont

Has it done this ever since the refurb? Did you replace the sender? If so, there may be a mismatch. Just one more thing to consider.
Martin

Martin, the car has undergone a full and detailed refurb - every item! The sender and gauge were both from the same original as aquired, though that doesn't mean long term they are an original matched pair. I don't know the car's detailed history.

Bob, I could do that but suspect there is another step I can try before that. I cannot remember the proceedure but have an idea that if I earth one of the gauge terminals it will mimic a full travel of the sender rheostat and would show a full tank if the gauge is working correctly, and the fault would be with the sender. Is that not right?

If however it only deflects to half full then it must be the gauge which is out of kilter.
GuyW

The article here http://www.midgetandspriteclub.com/wp-content/uploads/2019/10/newsletter201704.pdf on page 8 may be useful as it gives the resistance of the sender unit and details of the operation. That should allow the sender unit to be checked and help isolate where the problem is.
David Billington

Thanks David. I had forgotten about that MASCot article. October 2019
Useful
GuyW

Why does the magazine 1st page say April 2017 which would match the 201704 in the filename.
David Billington

Haven't looked it up yet David. I will check through my hard copies. I quoted the date from the link you posted 2019/10 without opening the link as my computer anti-viral didn't like it. Perhaps 'my' date was the file upload date and copy is April 2017.
GuyW

Quite right David. Its the April 2017 issue.
October 2019 must be an upload date.
I should have guessed it would be an excellent John Davies article. Now all I have to do is understand the words!





GuyW

HI Guy.

"-- if I earth one of the gauge terminals it will mimic a full travel of the sender rheostat and would show a full--".

Do the OPPOSITE.

B is for Battery.
T is for TANK. That T terminal, on the back of the fuel gauge, goes to the sender unit. DISCONNECT THAT, with the ignition on, and the gauge should read FULL.

If you GROUND the T terminal, simulating an empty tank, then the gauge should read EMPTY.

Ignition on.
EMPTY TANK.
Sender resistance near ZERO.
MAXIMUM CURRENT FLOW.
Highest voltage measured.
GAUGE READS EMPTY.

You probably know this. The fuel gauge is actually a voltage reading meter, calibrated to record the amount of petrol in the tank. The parallel resistor circuit, is a shunt circuit. The variable resistor in this, is the tank sender. That varies the current flow, and hence the voltage developed across the left coil in the schematic in this picture.

Ignition off.
Thus no current flow. No voltage developed across the left coil circuit. The needle rests at EMPTY position, only by gravity, no return spring inside.

No point writing more here.

You should follow this link to MGA GURU. SAME FUEL GAUGE AND SENDER, PRE voltage stabiliser.

IMO, It's the Heinekin of explanations, of how the gauge and sender work, and how to fix them when they don't.

http://mgaguru.com/mgtech/electric/fg_01.htm


Note:
Being fair, I was unable to download the PDF from the other link David B posted, so maybe that's just as good.




anamnesis

And, HERE'S the bit you wondered about, but unless someone has fiddled before, it seems unlikely you'd need to do this.


"Moving the magnets to different distances from the gauge armature changes their pull, so there is a means to calibrate the instrument, which is the subject of the next lesson."
anamnesis

Thanks Anam. I need to get the meter out and do some checking. I am mildly dyslexic when it comes to electronics jargon. Or at least the jargon doesn't come naturally to me,though I usually get there in the end. Words like "shunt" relate to faulty brakes in my mind, not voltages and current!
GuyW

A shunt on railways, shunts/diverts a train to a siding, or round the mainline.

An electrical shunt does similar for current, diverting a portion of the current, via a lower resistance parallel path.
anamnesis

Yep, I get that Anam. But it's not the first thing I think of! Indeed, talk about railway engines shunting in sidings and I also think it means pushing.

Twice today, on a 25 mile trip in the Frogeye, the car behind had to brake hard enough to get a chirrup from their tyres to avoid a shunt. It's not as if I was stopping hard in either case. My suspicion is that as our cars are small they appear to be further away than they really are. Like small cows in a big field ;-)
They take people by surprise!

Incidentally, these early gauges are designed to respond to the current, not a voltage reading device. It's why they don't require a voltage stabiliser, unlike with the later cars.
GuyW

As Anam said, with tank sender disconnected and ignition ON, gauge should read FULL. If not check it is earthing from its casing. If earth is OK AND gauge reads low, there is a gauge fault. If gauge reads full then suspect the sender. Sender resistance can be measured between the T wire (wire disconnected) and earth. Full tank should give 80 ohms or so. If lower than 80, it suggests float is sitting too low - might have fuel in it.
Bill Bretherton

Thanks Bill
Some testing now.

The needle goes right to stop peg on full when I disconnect sender wire. Earth is good - a dedicated wire direct to a body screw at the top of the A post. Other terminals are clean and tight. I had done a decent job with my wiring!

With tank sender wire disconnected I am only getting 49.2ohms so either it's a fault on the sender, or I haven't got as much fuel in there as I thought! I filled it on Friday, and thought I had brimmed it but maybe the pump had been to fast and splashing back had caused it to cut off. Anyway, dipstick first and then it's tank out time but I will need to drive around a bit to use some fuel up first.
GuyW

Guy

It really sounds like a sender issue.Even if you thought you brimmed the tank and the petrol pump cut off its very likely to be more than 1/2 fuel tank full. Does your tank have a drain plug at the bottom to avoid driving it around and using up the valuable petrol. Its now £1.66 /ltr in London now for E10 . £1.90 for E5
Bob Beaumont

Checked with a dipstick. It's definitely full so it's the sender. I also discover that when that full it's also leaking fuel off the top of the tank when it swills around. It will be coming from the sender gasket so another reason to sort it!
No, it doesn't have a drain plug. New tank, one of those Canadian ones. Well made.
GuyW

Guy, when I replaced my SU pump because I thought it was faulty I hung onto it 'just in case'. It has come in really useful for pumping fuel in and out on several occasions. I added a short length of copper pipe to the end of PVC tube to weight it so it sat right on the bottom and it empties the tank out almost completely, only a few dregs left when I removed the tank.
C Mee

"one of those Canadian ones. Well made" - I put one of those in new to replace the tank that was on the car when I got it. I forget why it needed replacing now, leak at outlet union IIRC, be in the Archives. I was a bit dismayed to find the tank I took out was one of those Canadian ones, only difference was new one had a paper label for part number old one was pressed in the metal.

Anyway, I had a leak one time at top of the tank, I could see evidence of the runs on the side of the tank, just about or over a year from buying the tank and I bought a new gasket and other parts, borrowed a mate's sender spanner kit ready to drop the tank and rewire the fuel pump and sender but the leak stopped so I didn't bother. That was about 3 years ago and the leak hasn't returned, don't know why (don't care) perhaps the rubber sender seal decide to swell or something, no doubt yours is cork or leather from an old (or used) pair of old boots, just my experience, I have a few with old cars.

The irony is one of the reasons I wanted a later Spridget was for the extra gallon fuel tank and when I got the Midget I found it didn't even hold 6 galls let alone 7 as the tank was replaced. I have to carry a full spare (5 litres, plastic) petrol can in the boot, it is the law of the household and I fear the punishment of breaking that law (even if sometimes I know the can is there but it's empty).



Nigel Atkins

Colin, that's a good idea. I have a spare pump. I bought two as a job lot on eBay. An SU points one that just needed cleaning and adjusting, and a brand new still in its box Hardi. £10 for the pair (they were poorly listed!) I will leave Nigel to tell you which is fitted in the car.

Nigel, I liked the Canadian tank. Stainless and with a nice continuous electric rolled weld. The only down side is that lack of a drain plug.

That Sealey kit looks elaborate! I just use a screwdriver. Maybe that's why my sender unit hasn't sealed to its gasketted and Hylomared joint properly!
GuyW

Guy,
I’ve got the same job to do sometime soon to a ‘B’.
I’ve got a clear plastic tube with a rubber bellows type pump to start a siphon ( better than trying the mouth suction method !). Sort of thing used on outboard engines. When I bought it I think it was sold for fluid transfers - bigger bore then fuel tube.

R.
richard b

This thread was discussed between 11/03/2022 and 13/03/2022

MG Midget and Sprite Technical index

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